CA2189360C - Nox sensor - Google Patents
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- CA2189360C CA2189360C CA002189360A CA2189360A CA2189360C CA 2189360 C CA2189360 C CA 2189360C CA 002189360 A CA002189360 A CA 002189360A CA 2189360 A CA2189360 A CA 2189360A CA 2189360 C CA2189360 C CA 2189360C
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- 239000007784 solid electrolyte Substances 0.000 claims abstract description 17
- 230000007423 decrease Effects 0.000 claims abstract description 11
- GEYXPJBPASPPLI-UHFFFAOYSA-N manganese(III) oxide Inorganic materials O=[Mn]O[Mn]=O GEYXPJBPASPPLI-UHFFFAOYSA-N 0.000 claims abstract description 3
- 229910044991 metal oxide Inorganic materials 0.000 claims abstract description 3
- 150000004706 metal oxides Chemical class 0.000 claims abstract description 3
- 229910052751 metal Inorganic materials 0.000 claims description 4
- 239000002184 metal Substances 0.000 claims description 4
- 239000000956 alloy Substances 0.000 claims description 3
- 229910045601 alloy Inorganic materials 0.000 claims description 3
- 229910052737 gold Inorganic materials 0.000 claims description 3
- 229910052741 iridium Inorganic materials 0.000 claims description 3
- 229910052763 palladium Inorganic materials 0.000 claims description 3
- 229910052697 platinum Inorganic materials 0.000 claims description 3
- 229910052703 rhodium Inorganic materials 0.000 claims description 3
- 229910052707 ruthenium Inorganic materials 0.000 claims description 3
- 239000000919 ceramic Substances 0.000 claims description 2
- 229910052709 silver Inorganic materials 0.000 claims description 2
- 230000035945 sensitivity Effects 0.000 abstract description 6
- 229910052760 oxygen Inorganic materials 0.000 description 21
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 19
- 239000001301 oxygen Substances 0.000 description 19
- 239000007789 gas Substances 0.000 description 17
- 238000006243 chemical reaction Methods 0.000 description 11
- 239000000126 substance Substances 0.000 description 11
- MCMNRKCIXSYSNV-UHFFFAOYSA-N Zirconium dioxide Chemical compound O=[Zr]=O MCMNRKCIXSYSNV-UHFFFAOYSA-N 0.000 description 6
- 150000002500 ions Chemical class 0.000 description 6
- 239000007772 electrode material Substances 0.000 description 5
- 230000003197 catalytic effect Effects 0.000 description 4
- 238000002485 combustion reaction Methods 0.000 description 4
- 238000000034 method Methods 0.000 description 4
- 239000007787 solid Substances 0.000 description 4
- 238000003487 electrochemical reaction Methods 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 150000002823 nitrates Chemical class 0.000 description 3
- 150000002826 nitrites Chemical class 0.000 description 3
- 239000004065 semiconductor Substances 0.000 description 3
- 238000000926 separation method Methods 0.000 description 3
- 229910052727 yttrium Inorganic materials 0.000 description 3
- 229910017771 LaFeO Inorganic materials 0.000 description 2
- 241000877463 Lanio Species 0.000 description 2
- 229910002651 NO3 Inorganic materials 0.000 description 2
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 2
- 239000004020 conductor Substances 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 238000012544 monitoring process Methods 0.000 description 2
- SQGYOTSLMSWVJD-UHFFFAOYSA-N silver(1+) nitrate Chemical compound [Ag+].[O-]N(=O)=O SQGYOTSLMSWVJD-UHFFFAOYSA-N 0.000 description 2
- 238000005245 sintering Methods 0.000 description 2
- 229910001415 sodium ion Inorganic materials 0.000 description 2
- 238000004544 sputter deposition Methods 0.000 description 2
- 238000001947 vapour-phase growth Methods 0.000 description 2
- RUDFQVOCFDJEEF-UHFFFAOYSA-N yttrium(III) oxide Inorganic materials [O-2].[O-2].[O-2].[Y+3].[Y+3] RUDFQVOCFDJEEF-UHFFFAOYSA-N 0.000 description 2
- ODINCKMPIJJUCX-UHFFFAOYSA-N Calcium oxide Chemical compound [Ca]=O ODINCKMPIJJUCX-UHFFFAOYSA-N 0.000 description 1
- 229910002467 CrFe Inorganic materials 0.000 description 1
- 229910052688 Gadolinium Inorganic materials 0.000 description 1
- 206010021143 Hypoxia Diseases 0.000 description 1
- 229910017916 MgMn Inorganic materials 0.000 description 1
- 241001077660 Molo Species 0.000 description 1
- 239000002228 NASICON Substances 0.000 description 1
- 229910052779 Neodymium Inorganic materials 0.000 description 1
- 229910052772 Samarium Inorganic materials 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
- 229910009580 YMnO Inorganic materials 0.000 description 1
- 229910052769 Ytterbium Inorganic materials 0.000 description 1
- 238000002679 ablation Methods 0.000 description 1
- 229910052788 barium Inorganic materials 0.000 description 1
- 229910001422 barium ion Inorganic materials 0.000 description 1
- 229910000416 bismuth oxide Inorganic materials 0.000 description 1
- 229910052793 cadmium Inorganic materials 0.000 description 1
- 229910052791 calcium Inorganic materials 0.000 description 1
- 235000012255 calcium oxide Nutrition 0.000 description 1
- 239000000292 calcium oxide Substances 0.000 description 1
- CETPSERCERDGAM-UHFFFAOYSA-N ceric oxide Chemical compound O=[Ce]=O CETPSERCERDGAM-UHFFFAOYSA-N 0.000 description 1
- 229910000422 cerium(IV) oxide Inorganic materials 0.000 description 1
- 238000005234 chemical deposition Methods 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 238000006477 desulfuration reaction Methods 0.000 description 1
- 230000023556 desulfurization Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- TYIXMATWDRGMPF-UHFFFAOYSA-N dibismuth;oxygen(2-) Chemical compound [O-2].[O-2].[O-2].[Bi+3].[Bi+3] TYIXMATWDRGMPF-UHFFFAOYSA-N 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000003411 electrode reaction Methods 0.000 description 1
- 239000003792 electrolyte Substances 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000007737 ion beam deposition Methods 0.000 description 1
- 238000007733 ion plating Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000005289 physical deposition Methods 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- SBYHFKPVCBCYGV-UHFFFAOYSA-N quinuclidine Chemical compound C1CC2CCN1CC2 SBYHFKPVCBCYGV-UHFFFAOYSA-N 0.000 description 1
- 238000006479 redox reaction Methods 0.000 description 1
- 238000006722 reduction reaction Methods 0.000 description 1
- 238000007650 screen-printing Methods 0.000 description 1
- 229910001961 silver nitrate Inorganic materials 0.000 description 1
- 229910052712 strontium Inorganic materials 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- 238000001771 vacuum deposition Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4075—Composition or fabrication of the electrodes and coatings thereon, e.g. catalysts
Abstract
The present invention is concerned with a NOx sensor consisting essentially of first and second electrodes in contact with a solid electrolyte, the sensor being capable of detecting a concentration of NOx in a gas by converting an amount of NOx concentration into an electromotive force level between the first and second electrodes. The electromotive force linearly increases in response to an increase, in terms of a logarithm, of NO2 concentration and linearly decreases in response to an increase, in terms of a logarithm, of NO concentration. At least the first electrode is composed of a metal oxide selected from the group consisting of Mn2O3, CdFe2O4, ZnFe3O4, CrFe2O4, Cr2CoO4 , CdGaMnO4 and YFeMnO4. The NOx sensor according to the invention has a good sensitivity without being affected by the concentration of CO2 and can detect NOx concentration in an exhaust gas at a temperature above 600.degree.C.
Description
NOx Sensor Field of the Invention The present invention relates to a sensor to detectNOx concentration in an exhaust gas generated from combustion furnace, automobile engine, etc.
Prior Art Continuous monitoring, feedback control of the combustion condition based on the monitoring result or the optimalization of the control of the desulfurization equipment have been examined to reduce NOx exhausted from combustion furnace, automobile engine, etc. Therefore, a smallsized solid and simple solid type sensor with high sensitivity is needed to be used for the control.
Especially, for the feedback of the combustion condition, a sensor which works stablely in a high temperature exhausted gas of several hundreds °C-.
A semiconductor type sensor utilizing a phenomena that the electric resistance of a gas sensor composed of oxide semiconductor changes in proportion to the concentration of NOx or a solid electrolyte sensor measuring the electromotive force caused-by the difference in the partial pressure of a gas at each electrodes which are formed on an ion conductive solid electrode are the representative sensors being proposed up to now.
However, there is a drawback in a semiconductor _ 2 _ 1 type sensor that enough sensitivity can not be obtained at a temperature above 500°C where the physical absorption of the test gas does not occur, since it utilize the phenomena of the change of electric resistance of the gas sensor caused by the physical absorption.
On the other hand, a solid electrolyte sensor, using AgI or RbAg,Is as a solid electrolyte and an electrode coated by silver nitrate is disclosed (The Japanese Laid-open Patent Publication Sho61-184450).
This sensor is a concentration cell type, wherein Ag ion in the nitrate migrates in the solid electrolyte by the difference of NOx concentration between the electrodes and causes the eletromotive force which complys with Nernst formula and detects NOx concentration by the measurement of said electromotive force. A concentration cell type sensor based on the same principle using NASICON
( Na, Zr, Si, PO1 , ) as a solid electrolyte and NaNO, as electrodes is disclosed. (Chemistry Letters, vol.l, p.587 590 (1992)) Furthermore, a concentration cell using Na ion conductive ~ /,B " alumina orb /~ " alumina, in which Na ion is replaced by Ba ion and using Ba(N0,), or mixture of NaNO, and Ba(N0,), as an electrode is disclosed in Denki Kagaku, vo1.59, (1991) p.465 N 472.
In these sensors which detect NOx concentration by the measurement of the electromotive force which complys with Nernst formula, nitrates or nitrites are used as an electrode material and then the durable temperature of 1 the sensor is limited by the melting point of nitrates or nitrites. Even when Ba(N0,),, having the highest melting point is used as an electrode, the durable temperature is below 592'C .- Furthermore, due to the diliquescence property of nitrates and nitrites, performance and stability of the sensor were not enough when used in the test gas containing water vapor.
For resolving above issues, a NOx sensor using an oxide of 6a Group element and oxides of perovskite or pseudo perovskite as an electrode material is proposed.
These sensors have a good thermnal durability due to the high melting point and decomposition temperature of the oxides. --However, the response of the NOx sensor of this type is different from that of the sensor using nitrate as an electrode-which complys with Nernst formula and depends on the change of electromotive force caused by the catalytic activity of the electrode for the test gas and oxidation reduction reactions on the electrode and hence the performance of the sensor depends largely on the oxides.--FOr example, electrodes using oxides of perovskite or pseudo perovskite structure containing Sn or Cu respond not only to NOx but also CO, and there are issues that the exact detection of NOx concentration is not possible, since it is affected-very much by the CO, concentration. And a sensor using oxides of 6a Group element and perovskite structure containing Ti as an electrode works well around 500 °C , but its sensitivity to NOx rapidly decreases above 600°C and there are issues in the reduction of resolution and accuracy of NOx concentration.
Disclosure of the Present Invention It is an object of the present invention to provide a NOx sensor which does not respond to COz and has a good sensitivity to NOx at a temperature above 600°C.
In accordance with the present invention, there is provided a NOx sensor consisting essentially of first and second electrodes in contact with a solid electrolyte, the sensor being capable of detecting a concentration of NOx in a gas by converting an amount of NOx concentration into an electromotive force level between the first and second electrodes. The electromotive force linearly increases in response to an increase, in terms of a logarithm, of N02 concentration and linearly decreases in response to an increase, in terms of a logarithm, of NO
concentration. At least the first electrode is composed of a metal oxide selected from the group consisting of 2 0 Mn203 , CdFe204 , ZnFe304 , CrFe204 , CrzCo04 , CdGaMn04 and YFeMn04 .
In a more concrete form of the NOx sensor of the present invention, zirconia (Zr02-M203 or Zr02-MO, M is Yb, Cd, Nd, Ca, Y, Mg, Hf) bismuth oxide (bi203-M203 or MO
or M205, M is Y, Gd, Nb, W, Sr, Ba) and ceria oxide (Ce02-M203 or MO2, M is Y, Sm) are used as a solid electrolyte.
A solid electrolyte can be either a separation wall structure which separates the test gas to be detected of NOx concentration from the constant atmospheric gas such as air,a plate or rod shape. In case a solid electrolyte is a separation wall structure, the first and second electrode are placed on each surface of the wall and in 1 the case that a solid electrolyte is not a separation wall structure, the first and second electrodes are placed at any place on the solid electrolyte. -The first electrode to be formed on an electrolyte is composed either of oxides of Mn, Fe, Co and Ni or a substance containing said oxide. Furthermore, the first electrode is composed either of hybridized oxides expressed by ABO, , AB, 04 , A, B0, and ACB, or a substance containing said hybridized-oxides. Here, B is an element selected from 7a or 8a Group and A and C are elements selected from 2A, 3A, 4A, 5A, 6A, 8, 1B, 2B, 3B, 4B, 5B
Group and lanthanide. A part of A and C element of the hybrid oxides can be replaced by an element having similar ion radius or an element with same or close in valency and furthermore a part of B of said hybrid oxides can be replace3 by an element having similar ion radius or an element with same or close in valency. The oxygen content expressed by the chemical formula can be a stoichiometric value including a non-integral number caused by the oxygen deficiency. These hybrid oxides can be a mixture of.a single oxide of the element composing of hybrid oxides so far as hybrid oxides are the main component. If required, an electric collector composed of rare metals such as Pt, Au, Pd, Ir, Rh, Ru or its alloy as an electro-conductive material can be formed to keep electrical connection.
1 The first electrode of an oxide or a hybrid oxide is formed on a solid electrolyte by coating using screen printing, etc., and sintering and also it can be formed by physical deposition method such as vacuum deposition, lasor ablation, ion beam deposition and ionplating method and chemical deposition method such as chemical vapour phase deposition and plasma chemical vapour phase deposition.
The second electrode is composed of a substance comprising rare metals, such as Pt, Ag, Au, Pd, Ir, Rh and Ru, its alloy, electro-conductive ceramics, for example, oxides of perovskite structure expressed by ABO,, such as LaCo03, LaNiO, , LaFeO, , wherein a part of A or' B site can be replaced by an element such as Sr, an oxide having a structure of K,NiF, and a substance which can be an oxygen electrode such as La,CuO~ or a substance which can give a definite chemical potential to oxygen. However, in a sensor wherein both the first and second electrode are exposed in the test gas, it is reguired that the change of the electrical potential caused at the first electrode should not be cancelled by the change of the electrical potential caused by the second electrode and it is preferred that it is composed of a material which does not cause a change of electrical potential for NO and NO, or a material which cause a change of electrical potential in an opposite direction to that caused at the first electrode.
_,_ 1 In an electromotive forcetype sensor, wherein a pair of-electrodes is formed on a solid electrolyte and the difference of the electrical potential caused by the difference in the chemical potentialof the electrodes, the first electrode which works as a detecting electrode, reacts with the test gas existing on the surface of the electrode and as a result, the difference in the electrical-potential is caused by the change of the chemical- potential of the ion conductive carrier of a solid electrolyte-to the chemical potential of the other electrode, which complys with the-Nernst formula.
However, the change of the electromotive force in the present invention does not comply with the Nernst formula, because the number of the electrons obtained from the slope of the dependency of the electromotive force on the concentration and temperature are not integral numbers. In a NOx sensor according to the present invention, the change of the electromotive force is different for NOz and N0. The electromotive form increases depending on the increase of the concentration of NO, and decreases depending on the increase of NO
concentration. Judging from-the change of the electromotive force corresponding-to the change of the concentration of the test gas, it is thought that the reducti-on reaction of the test gas occurs on the electrode when the electromotive force increases and oxydation reaction occurs-on the electrode when the _$_ 1 electromotive foxce decreases. Furthermore, the dependency of the electromotive force on the concentration of NOx is different in the presence and absence of oxygen, wherein the slope of the dependency of the electromotive force on NOx concentration in the presence of oxygen is larger than that in the absence of oxygen;
that is oxygen is involved in the electrode reaction.
Namely the response of the sensor according to the present invention is obtained by the following reactions.
On the first electrode, an electrochemical reaction between oxygen-and NOx is concomitantly occurs and the change of the electromotive force is caused by the hybrid electropotential of the local battery formed on the first electrode. Namely, a hybrid-electric potential is caused for NOz by the following reactions on the first electrode NO~ + 2e -~ NO + ~- (1) O'- -~ % 0~+ 2e (2) and a hybrid electric potential is caused for NO by the following reactions on the first electrode '/z Os + 2e -> O' - (3 ) NO + O' --~ NO, + 2e ( 4 ) To obtain the electromotive force by the hybrid electric potential, it is necessary that the two reactions occur concomitantly on the electrode either for NO, and NO and the hybrid electric potential can not be obtained;
if it is active for a reaction and not active for another reaction. Since the NOx sensor according to the present _ g _ 1 invention is composed of an oxide of 7a or 8 Group element,. which is used as an oxygen electrode in a fuel battery or oxygen sensor and hybridoxides containing 7a or 8 Group element, it is possible to act as an oxygen g electrodeand has an enough activity for the reactions of (2) and (4). Furthermore, oxides of 7a and 8 Group element and hybrid oxides containing 7a and 8 Group element also have a high catalytic activity for NOx and are active for the reactions of (1) and (2). Namely, since it-is active both for oxygen and NOx, the change of the electromotive force is obtained by the hybrid electric potential caused by-the electrochemical reaction between oxygen and NOx which occurs- concomitantly on the first electrode and occurs also at 600°C , and hence a good - performance is obtained. On the-other-hand, it is thought that a hybrid electric potential is caused for CO~ .
gas by the same reaction for NO,. However, the catalytic activity of the electrode according to the present invention is low for COz and electro-chemical reaction 2p between oxygen and COs does not occur concomitantly on the first electrode and hence it does not respond to CO~
Brief Explanation of the Figure Fig. 1 is a cross section of an example of a NOx sensor according to the present invention.
Fig. 2_is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention.
Fig. 3 is a figure showing the dependency of the change of the electromotive force on the NOx concentration in an example of a NOx sensor according to the present invention.
Fig. 4 is a figure showing the dependency of the change of the electromotive force on the C0, concentration.
Fig. 5 is a figure showing the dependency of the change of the electromotive force on the NOx sensor according to the present invention.
Fig. 6 is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention.
Fig. 7 is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention.
Preferred Embodiment of the Present Invention Example 1 Fig. 1 shows a cross section of a NOx sensor in an example according to the present invention. A solid electrolyte 1 can be any oxgen ion conductive material, however, in view of thermal stability and resistance, zirconia, wholly or partially stabilized by~yttria, calcia or magnecia, etc. is preferred. In this example, 1 zirconia stabilized by8 molo of yttria was used. On one surface of a plate solid electrolyte, the first electrode 2 and second electrode 3 are placed:- The first electrode 2 is composed of oxides of 7a or 8a Group element. The first electrode was prepared by sputtering using oxide as a target. After preparation of a film by sputtering, heat treatment was conducted for 1 hr at 900'C in the air. The second electrode 3 is composed of an electrode which does not respond to NOx and Pt is used in this case. Thefirst electrode 2 and thesecondelectrode 3 is a so called gas electrode and formed as a porous electrode. An electric collector 4-of Pt is placed on the first electrode 2 and the cable 5-and 6 of the second electrode 3 is. connected with the measurement circuit.
The change of theelectromotiveforce at the introduction of 100ppm of N0~ and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600°C as a standard is shown in Table 1. The NOx sensor, whichever oxides are used, ZO responds to NOz resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force. Among oxides, the sensor using Co as the first electrode showed the heighest sensitivity.
Figs. 2 and 3 show the dependency of the electromotive force ofa NOx sensor using oxides of Co as the first electrode on NOx concentration. Fig. 4 shows the dependency of the electromotive force of a NOx sensor 1 using oxides of Co as the first electrode on C0, concentration at 600°C . The sensor does not respond to C0, and the electromotive force does not change even when COz concentration changes. The dependency of the electromotive force on NOx concentration and the responce characteristic to COz were same for the sensor using other oxides shown in this example.
Table 1 The Change of the Electromotive Force corresponding to 100ppm NOz and 500ppm No The Change of Electrode Material the Electromotive Force (mV) 100ppm N0, 500ppm NO
Mn, 0, 16 . 9 -10 . 5 Fe, 0,. 28.7 -11.5 , Co304 64.3 -38.1 Ni0 43.8 - -10.1 Eample 2 A NOx sensor was prepared using hybrid oxides expressed by the same method as explained in Example 1.
Hybrid oxides expressed by ABO, used as the first 1 electrode ware prepared by sintering a mixture of a single oxide of each element composing of hybrid oxides.
It was confirmed by the X-ray diffraction that these hybrid oxides are composed of a single phase or a mixed phase of oxides of either A or B. The change of electromotive force at the introducing of 100ppm of NOz and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600 °C as a standard is shown in Table 2. A NOx sensor using hybrid oxides expressed by ABO, as the first electrode responds to N0, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of N0~ and NO was proportional to the logarism of the concentration. Furthermore, any of the sensors did not respond to CO,. Fig. 5 shows the dependency of the electromotive force of a NOx sensor at 600 °C on NOx concentration.
1 Table 2 The Change of the Electromotive Force corresponding to 100ppm N0~ and 500ppm NO
The Change of l the Electromotive t Force (mV) d t i l E 100ppm N0, 500ppm NO
ec ro e Ma er a MgMnO, 37.4 -18.2 SrMnO, 66.1 -11.4 CaMnO, 30.5 -14.5 BaMnO, 43.6 -12.8 YMnO, 25.3 -11.8 CrMnO, 32.2 -10.0 CoMn03 36.8 -17.4 NiMnO, 52.4 -25.7 AgMnO, 32.1 -13.5 ZnMnO, 57.5 -35.6 LaMnO, 44.8 -25.7 NdMnO, 35.4 -18.7 SmMnO, 39.0 -10.7 (YSr)MnO, 22.6 -10.5 GaFeO, 21.8 - 9.2 LaFeO, 49.3 -19.8 LaCo03 45.2 -20.1 LaNiO, 22.4 -11.2 SrNiO, 28.6 -10.9 Eample 3 A Nox sensor was preapred using hybrid oxides exp essed by AB, O, as the first electrode by the same method as shown in Example 2. The change of the electromotive 1 force at the introduction of 100ppm of NO, and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600 °C as a standard is shown in Table 3. In this example, it was also confirmed that the sensor responds N0, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO, and NO
was proportional to the logarism of the concentration.
AnY sensordid not responds to C0, at the first electrode.
In Fig. 6, the dependency of the electromotive force of a NOx sensor using CdMn,O, as the first electrode on NOx concentration at 600°C is shown.
1 Table 3 The Change of the Electromotive Force corresponding to 100ppm NOz and 500ppm No The Change of e Electromotive t th (mV) l Force d i l e Ma 100ppm N0, 500ppm NO
er E
ectro a MgMn, 0, 3 5 . 5 -16 :1 TiMnz 0, 25. 4 -11.8 CrMn, 0, 28 .2 -17 .1 CoMnz04 29.7 -15.9 CdMn, 0, 4 7 . 9 -3 0 . 6 (CdAl )Mn, 0, 46.8 -25.6 ( CdAl ) ( MnCr 10 . 8 -19 . 4 ), 0, CdFe, 0, 18 . 0 -10 . 2 ZnFe, 0, 34.5 -18 .6 CrFe~ 0, 26.7 -12.3 Example 4 A NOx sensor was prepared using hybrid oxides expressed by A=B0, as the first electrode by the same method as explained in Example 2 and the change of the electromotive force at the introduction of 100ppm of NOz and 500ppm of NO taking the value of the electromotive 1 force under the atmospheric circumstance of 40 oxygen at 600 °C as a standard is shown in Table 4. In this example, it was also confirmed that the sensor responds the N0, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of N0, and NO was proportional to the logarism of the concentration. In Fig. 7, the dependency of the change of electromotive force at 600°C of a NOx sensor using La,MnO, as the first electrode is shown.
Table 4 The Change of the Electromotive Force corresponding to 100ppm N0, and 500ppm NO
--The Change of d the Electromotive l Force (mV) t i l E 100ppm N0, 500ppm NO
ec ro e Mater a Caz MnO, 33 .1 -14 .1 Mgz MnO~ 29. 5 -12.2 Fe, MnO, 33 . 8 -14. 0 Ni, Mn04 56 . 7 -10 . 2 Zn~ Mn04 50 . 0 -17 . 6 Alz MnO, 28.6 -11. 9 Sb, Mn04 58 .2 -10. 4 Lay MnO, 44.8 -25.7 Cr2 Co04 5 5 .1 -18 . 8 1 Example 5 A NOx sensor was prepared using hybrid oxides expressed by ABCO, as the first electrode by the same method as explained in Example 1 and the change of the electromotive force at the introduction of IOOppm of N0, and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600 'C as a standard is shown in Table 5.
In this example it was also confirmed that the sensor responds to NO, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO, and NO was proportional to the logarism of the concentration. Either of the electrodes did not respond to CO,.
Table 5 The Change of the Electromotive Force corresponding to 100ppm NO, and 500ppm NO
. . ~_ . -.:,.~:, _,.:. ..-.
:.. ~ _ ...
The Change of the Electromotive Force (mV) Electrode Material 100ppm NO, 500ppm NO
CdGaMnO. 44.7 -26.3 yFeMnO~ 40.3 -16.6 1 Possible Utilization of the Invention in the Industry Since oxides or hybrid oxides which have no catalytic activity for CO, are used as the electrode material, it is possible to detect NOx concentration correctly without being affected by the fluctuation of CO, concentration. Furthermore, those oxides or hybrid oxides are active both for oxygen and NOx and hence the sensor looks at a temperature above 600 °C and detects NOx concentration with high accuracy.
Prior Art Continuous monitoring, feedback control of the combustion condition based on the monitoring result or the optimalization of the control of the desulfurization equipment have been examined to reduce NOx exhausted from combustion furnace, automobile engine, etc. Therefore, a smallsized solid and simple solid type sensor with high sensitivity is needed to be used for the control.
Especially, for the feedback of the combustion condition, a sensor which works stablely in a high temperature exhausted gas of several hundreds °C-.
A semiconductor type sensor utilizing a phenomena that the electric resistance of a gas sensor composed of oxide semiconductor changes in proportion to the concentration of NOx or a solid electrolyte sensor measuring the electromotive force caused-by the difference in the partial pressure of a gas at each electrodes which are formed on an ion conductive solid electrode are the representative sensors being proposed up to now.
However, there is a drawback in a semiconductor _ 2 _ 1 type sensor that enough sensitivity can not be obtained at a temperature above 500°C where the physical absorption of the test gas does not occur, since it utilize the phenomena of the change of electric resistance of the gas sensor caused by the physical absorption.
On the other hand, a solid electrolyte sensor, using AgI or RbAg,Is as a solid electrolyte and an electrode coated by silver nitrate is disclosed (The Japanese Laid-open Patent Publication Sho61-184450).
This sensor is a concentration cell type, wherein Ag ion in the nitrate migrates in the solid electrolyte by the difference of NOx concentration between the electrodes and causes the eletromotive force which complys with Nernst formula and detects NOx concentration by the measurement of said electromotive force. A concentration cell type sensor based on the same principle using NASICON
( Na, Zr, Si, PO1 , ) as a solid electrolyte and NaNO, as electrodes is disclosed. (Chemistry Letters, vol.l, p.587 590 (1992)) Furthermore, a concentration cell using Na ion conductive ~ /,B " alumina orb /~ " alumina, in which Na ion is replaced by Ba ion and using Ba(N0,), or mixture of NaNO, and Ba(N0,), as an electrode is disclosed in Denki Kagaku, vo1.59, (1991) p.465 N 472.
In these sensors which detect NOx concentration by the measurement of the electromotive force which complys with Nernst formula, nitrates or nitrites are used as an electrode material and then the durable temperature of 1 the sensor is limited by the melting point of nitrates or nitrites. Even when Ba(N0,),, having the highest melting point is used as an electrode, the durable temperature is below 592'C .- Furthermore, due to the diliquescence property of nitrates and nitrites, performance and stability of the sensor were not enough when used in the test gas containing water vapor.
For resolving above issues, a NOx sensor using an oxide of 6a Group element and oxides of perovskite or pseudo perovskite as an electrode material is proposed.
These sensors have a good thermnal durability due to the high melting point and decomposition temperature of the oxides. --However, the response of the NOx sensor of this type is different from that of the sensor using nitrate as an electrode-which complys with Nernst formula and depends on the change of electromotive force caused by the catalytic activity of the electrode for the test gas and oxidation reduction reactions on the electrode and hence the performance of the sensor depends largely on the oxides.--FOr example, electrodes using oxides of perovskite or pseudo perovskite structure containing Sn or Cu respond not only to NOx but also CO, and there are issues that the exact detection of NOx concentration is not possible, since it is affected-very much by the CO, concentration. And a sensor using oxides of 6a Group element and perovskite structure containing Ti as an electrode works well around 500 °C , but its sensitivity to NOx rapidly decreases above 600°C and there are issues in the reduction of resolution and accuracy of NOx concentration.
Disclosure of the Present Invention It is an object of the present invention to provide a NOx sensor which does not respond to COz and has a good sensitivity to NOx at a temperature above 600°C.
In accordance with the present invention, there is provided a NOx sensor consisting essentially of first and second electrodes in contact with a solid electrolyte, the sensor being capable of detecting a concentration of NOx in a gas by converting an amount of NOx concentration into an electromotive force level between the first and second electrodes. The electromotive force linearly increases in response to an increase, in terms of a logarithm, of N02 concentration and linearly decreases in response to an increase, in terms of a logarithm, of NO
concentration. At least the first electrode is composed of a metal oxide selected from the group consisting of 2 0 Mn203 , CdFe204 , ZnFe304 , CrFe204 , CrzCo04 , CdGaMn04 and YFeMn04 .
In a more concrete form of the NOx sensor of the present invention, zirconia (Zr02-M203 or Zr02-MO, M is Yb, Cd, Nd, Ca, Y, Mg, Hf) bismuth oxide (bi203-M203 or MO
or M205, M is Y, Gd, Nb, W, Sr, Ba) and ceria oxide (Ce02-M203 or MO2, M is Y, Sm) are used as a solid electrolyte.
A solid electrolyte can be either a separation wall structure which separates the test gas to be detected of NOx concentration from the constant atmospheric gas such as air,a plate or rod shape. In case a solid electrolyte is a separation wall structure, the first and second electrode are placed on each surface of the wall and in 1 the case that a solid electrolyte is not a separation wall structure, the first and second electrodes are placed at any place on the solid electrolyte. -The first electrode to be formed on an electrolyte is composed either of oxides of Mn, Fe, Co and Ni or a substance containing said oxide. Furthermore, the first electrode is composed either of hybridized oxides expressed by ABO, , AB, 04 , A, B0, and ACB, or a substance containing said hybridized-oxides. Here, B is an element selected from 7a or 8a Group and A and C are elements selected from 2A, 3A, 4A, 5A, 6A, 8, 1B, 2B, 3B, 4B, 5B
Group and lanthanide. A part of A and C element of the hybrid oxides can be replaced by an element having similar ion radius or an element with same or close in valency and furthermore a part of B of said hybrid oxides can be replace3 by an element having similar ion radius or an element with same or close in valency. The oxygen content expressed by the chemical formula can be a stoichiometric value including a non-integral number caused by the oxygen deficiency. These hybrid oxides can be a mixture of.a single oxide of the element composing of hybrid oxides so far as hybrid oxides are the main component. If required, an electric collector composed of rare metals such as Pt, Au, Pd, Ir, Rh, Ru or its alloy as an electro-conductive material can be formed to keep electrical connection.
1 The first electrode of an oxide or a hybrid oxide is formed on a solid electrolyte by coating using screen printing, etc., and sintering and also it can be formed by physical deposition method such as vacuum deposition, lasor ablation, ion beam deposition and ionplating method and chemical deposition method such as chemical vapour phase deposition and plasma chemical vapour phase deposition.
The second electrode is composed of a substance comprising rare metals, such as Pt, Ag, Au, Pd, Ir, Rh and Ru, its alloy, electro-conductive ceramics, for example, oxides of perovskite structure expressed by ABO,, such as LaCo03, LaNiO, , LaFeO, , wherein a part of A or' B site can be replaced by an element such as Sr, an oxide having a structure of K,NiF, and a substance which can be an oxygen electrode such as La,CuO~ or a substance which can give a definite chemical potential to oxygen. However, in a sensor wherein both the first and second electrode are exposed in the test gas, it is reguired that the change of the electrical potential caused at the first electrode should not be cancelled by the change of the electrical potential caused by the second electrode and it is preferred that it is composed of a material which does not cause a change of electrical potential for NO and NO, or a material which cause a change of electrical potential in an opposite direction to that caused at the first electrode.
_,_ 1 In an electromotive forcetype sensor, wherein a pair of-electrodes is formed on a solid electrolyte and the difference of the electrical potential caused by the difference in the chemical potentialof the electrodes, the first electrode which works as a detecting electrode, reacts with the test gas existing on the surface of the electrode and as a result, the difference in the electrical-potential is caused by the change of the chemical- potential of the ion conductive carrier of a solid electrolyte-to the chemical potential of the other electrode, which complys with the-Nernst formula.
However, the change of the electromotive force in the present invention does not comply with the Nernst formula, because the number of the electrons obtained from the slope of the dependency of the electromotive force on the concentration and temperature are not integral numbers. In a NOx sensor according to the present invention, the change of the electromotive force is different for NOz and N0. The electromotive form increases depending on the increase of the concentration of NO, and decreases depending on the increase of NO
concentration. Judging from-the change of the electromotive force corresponding-to the change of the concentration of the test gas, it is thought that the reducti-on reaction of the test gas occurs on the electrode when the electromotive force increases and oxydation reaction occurs-on the electrode when the _$_ 1 electromotive foxce decreases. Furthermore, the dependency of the electromotive force on the concentration of NOx is different in the presence and absence of oxygen, wherein the slope of the dependency of the electromotive force on NOx concentration in the presence of oxygen is larger than that in the absence of oxygen;
that is oxygen is involved in the electrode reaction.
Namely the response of the sensor according to the present invention is obtained by the following reactions.
On the first electrode, an electrochemical reaction between oxygen-and NOx is concomitantly occurs and the change of the electromotive force is caused by the hybrid electropotential of the local battery formed on the first electrode. Namely, a hybrid-electric potential is caused for NOz by the following reactions on the first electrode NO~ + 2e -~ NO + ~- (1) O'- -~ % 0~+ 2e (2) and a hybrid electric potential is caused for NO by the following reactions on the first electrode '/z Os + 2e -> O' - (3 ) NO + O' --~ NO, + 2e ( 4 ) To obtain the electromotive force by the hybrid electric potential, it is necessary that the two reactions occur concomitantly on the electrode either for NO, and NO and the hybrid electric potential can not be obtained;
if it is active for a reaction and not active for another reaction. Since the NOx sensor according to the present _ g _ 1 invention is composed of an oxide of 7a or 8 Group element,. which is used as an oxygen electrode in a fuel battery or oxygen sensor and hybridoxides containing 7a or 8 Group element, it is possible to act as an oxygen g electrodeand has an enough activity for the reactions of (2) and (4). Furthermore, oxides of 7a and 8 Group element and hybrid oxides containing 7a and 8 Group element also have a high catalytic activity for NOx and are active for the reactions of (1) and (2). Namely, since it-is active both for oxygen and NOx, the change of the electromotive force is obtained by the hybrid electric potential caused by-the electrochemical reaction between oxygen and NOx which occurs- concomitantly on the first electrode and occurs also at 600°C , and hence a good - performance is obtained. On the-other-hand, it is thought that a hybrid electric potential is caused for CO~ .
gas by the same reaction for NO,. However, the catalytic activity of the electrode according to the present invention is low for COz and electro-chemical reaction 2p between oxygen and COs does not occur concomitantly on the first electrode and hence it does not respond to CO~
Brief Explanation of the Figure Fig. 1 is a cross section of an example of a NOx sensor according to the present invention.
Fig. 2_is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention.
Fig. 3 is a figure showing the dependency of the change of the electromotive force on the NOx concentration in an example of a NOx sensor according to the present invention.
Fig. 4 is a figure showing the dependency of the change of the electromotive force on the C0, concentration.
Fig. 5 is a figure showing the dependency of the change of the electromotive force on the NOx sensor according to the present invention.
Fig. 6 is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention.
Fig. 7 is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention.
Preferred Embodiment of the Present Invention Example 1 Fig. 1 shows a cross section of a NOx sensor in an example according to the present invention. A solid electrolyte 1 can be any oxgen ion conductive material, however, in view of thermal stability and resistance, zirconia, wholly or partially stabilized by~yttria, calcia or magnecia, etc. is preferred. In this example, 1 zirconia stabilized by8 molo of yttria was used. On one surface of a plate solid electrolyte, the first electrode 2 and second electrode 3 are placed:- The first electrode 2 is composed of oxides of 7a or 8a Group element. The first electrode was prepared by sputtering using oxide as a target. After preparation of a film by sputtering, heat treatment was conducted for 1 hr at 900'C in the air. The second electrode 3 is composed of an electrode which does not respond to NOx and Pt is used in this case. Thefirst electrode 2 and thesecondelectrode 3 is a so called gas electrode and formed as a porous electrode. An electric collector 4-of Pt is placed on the first electrode 2 and the cable 5-and 6 of the second electrode 3 is. connected with the measurement circuit.
The change of theelectromotiveforce at the introduction of 100ppm of N0~ and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600°C as a standard is shown in Table 1. The NOx sensor, whichever oxides are used, ZO responds to NOz resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force. Among oxides, the sensor using Co as the first electrode showed the heighest sensitivity.
Figs. 2 and 3 show the dependency of the electromotive force ofa NOx sensor using oxides of Co as the first electrode on NOx concentration. Fig. 4 shows the dependency of the electromotive force of a NOx sensor 1 using oxides of Co as the first electrode on C0, concentration at 600°C . The sensor does not respond to C0, and the electromotive force does not change even when COz concentration changes. The dependency of the electromotive force on NOx concentration and the responce characteristic to COz were same for the sensor using other oxides shown in this example.
Table 1 The Change of the Electromotive Force corresponding to 100ppm NOz and 500ppm No The Change of Electrode Material the Electromotive Force (mV) 100ppm N0, 500ppm NO
Mn, 0, 16 . 9 -10 . 5 Fe, 0,. 28.7 -11.5 , Co304 64.3 -38.1 Ni0 43.8 - -10.1 Eample 2 A NOx sensor was prepared using hybrid oxides expressed by the same method as explained in Example 1.
Hybrid oxides expressed by ABO, used as the first 1 electrode ware prepared by sintering a mixture of a single oxide of each element composing of hybrid oxides.
It was confirmed by the X-ray diffraction that these hybrid oxides are composed of a single phase or a mixed phase of oxides of either A or B. The change of electromotive force at the introducing of 100ppm of NOz and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600 °C as a standard is shown in Table 2. A NOx sensor using hybrid oxides expressed by ABO, as the first electrode responds to N0, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of N0~ and NO was proportional to the logarism of the concentration. Furthermore, any of the sensors did not respond to CO,. Fig. 5 shows the dependency of the electromotive force of a NOx sensor at 600 °C on NOx concentration.
1 Table 2 The Change of the Electromotive Force corresponding to 100ppm N0~ and 500ppm NO
The Change of l the Electromotive t Force (mV) d t i l E 100ppm N0, 500ppm NO
ec ro e Ma er a MgMnO, 37.4 -18.2 SrMnO, 66.1 -11.4 CaMnO, 30.5 -14.5 BaMnO, 43.6 -12.8 YMnO, 25.3 -11.8 CrMnO, 32.2 -10.0 CoMn03 36.8 -17.4 NiMnO, 52.4 -25.7 AgMnO, 32.1 -13.5 ZnMnO, 57.5 -35.6 LaMnO, 44.8 -25.7 NdMnO, 35.4 -18.7 SmMnO, 39.0 -10.7 (YSr)MnO, 22.6 -10.5 GaFeO, 21.8 - 9.2 LaFeO, 49.3 -19.8 LaCo03 45.2 -20.1 LaNiO, 22.4 -11.2 SrNiO, 28.6 -10.9 Eample 3 A Nox sensor was preapred using hybrid oxides exp essed by AB, O, as the first electrode by the same method as shown in Example 2. The change of the electromotive 1 force at the introduction of 100ppm of NO, and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600 °C as a standard is shown in Table 3. In this example, it was also confirmed that the sensor responds N0, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO, and NO
was proportional to the logarism of the concentration.
AnY sensordid not responds to C0, at the first electrode.
In Fig. 6, the dependency of the electromotive force of a NOx sensor using CdMn,O, as the first electrode on NOx concentration at 600°C is shown.
1 Table 3 The Change of the Electromotive Force corresponding to 100ppm NOz and 500ppm No The Change of e Electromotive t th (mV) l Force d i l e Ma 100ppm N0, 500ppm NO
er E
ectro a MgMn, 0, 3 5 . 5 -16 :1 TiMnz 0, 25. 4 -11.8 CrMn, 0, 28 .2 -17 .1 CoMnz04 29.7 -15.9 CdMn, 0, 4 7 . 9 -3 0 . 6 (CdAl )Mn, 0, 46.8 -25.6 ( CdAl ) ( MnCr 10 . 8 -19 . 4 ), 0, CdFe, 0, 18 . 0 -10 . 2 ZnFe, 0, 34.5 -18 .6 CrFe~ 0, 26.7 -12.3 Example 4 A NOx sensor was prepared using hybrid oxides expressed by A=B0, as the first electrode by the same method as explained in Example 2 and the change of the electromotive force at the introduction of 100ppm of NOz and 500ppm of NO taking the value of the electromotive 1 force under the atmospheric circumstance of 40 oxygen at 600 °C as a standard is shown in Table 4. In this example, it was also confirmed that the sensor responds the N0, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of N0, and NO was proportional to the logarism of the concentration. In Fig. 7, the dependency of the change of electromotive force at 600°C of a NOx sensor using La,MnO, as the first electrode is shown.
Table 4 The Change of the Electromotive Force corresponding to 100ppm N0, and 500ppm NO
--The Change of d the Electromotive l Force (mV) t i l E 100ppm N0, 500ppm NO
ec ro e Mater a Caz MnO, 33 .1 -14 .1 Mgz MnO~ 29. 5 -12.2 Fe, MnO, 33 . 8 -14. 0 Ni, Mn04 56 . 7 -10 . 2 Zn~ Mn04 50 . 0 -17 . 6 Alz MnO, 28.6 -11. 9 Sb, Mn04 58 .2 -10. 4 Lay MnO, 44.8 -25.7 Cr2 Co04 5 5 .1 -18 . 8 1 Example 5 A NOx sensor was prepared using hybrid oxides expressed by ABCO, as the first electrode by the same method as explained in Example 1 and the change of the electromotive force at the introduction of IOOppm of N0, and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4~ oxygen at 600 'C as a standard is shown in Table 5.
In this example it was also confirmed that the sensor responds to NO, resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO, and NO was proportional to the logarism of the concentration. Either of the electrodes did not respond to CO,.
Table 5 The Change of the Electromotive Force corresponding to 100ppm NO, and 500ppm NO
. . ~_ . -.:,.~:, _,.:. ..-.
:.. ~ _ ...
The Change of the Electromotive Force (mV) Electrode Material 100ppm NO, 500ppm NO
CdGaMnO. 44.7 -26.3 yFeMnO~ 40.3 -16.6 1 Possible Utilization of the Invention in the Industry Since oxides or hybrid oxides which have no catalytic activity for CO, are used as the electrode material, it is possible to detect NOx concentration correctly without being affected by the fluctuation of CO, concentration. Furthermore, those oxides or hybrid oxides are active both for oxygen and NOx and hence the sensor looks at a temperature above 600 °C and detects NOx concentration with high accuracy.
Claims (2)
1. A NOx sensor consisting essentially of first and second electrodes in contact with a solid electrolyte, said sensor being capable of detecting a concentration of NOx in a gas by converting an amount of NOx concentration into an electromotive force level between the first and second electrodes, wherein the electromotive force linearly increases in response to an increase, in terms of a logarithm, of NO2 concentration and linearly decreases in response to an increase, in terms of a logarithm, of NO concentration, and werein at least said first electrode is composed of a metal oxide selected from the group consisting of Mn2O3, CdFe2O4, ZnFe3O4, CrFe2O4, Cr2CoO4, CdGaMnO4 and YFeMnO4.
2. The NOx sensor of claim 1, wherein said second electrode is composed of a metal selected from the group consisting of Pt, Ag, Au, Pd, Ir, Rh and Ru, an alloy of said metal or an electroconductive ceramic.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP78427/1995 | 1995-03-10 | ||
| JP07842795A JP3524980B2 (en) | 1995-03-10 | 1995-03-10 | Nitrogen oxide sensor |
| PCT/JP1996/000537 WO1996028722A1 (en) | 1995-03-10 | 1996-03-06 | Nitrogen oxide sensor |
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|---|---|
| CA2189360A1 CA2189360A1 (en) | 1996-09-19 |
| CA2189360C true CA2189360C (en) | 2001-02-20 |
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| Application Number | Title | Priority Date | Filing Date |
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| CA002189360A Expired - Fee Related CA2189360C (en) | 1995-03-10 | 1996-03-06 | Nox sensor |
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| US (1) | US6019881A (en) |
| EP (1) | EP0759552B1 (en) |
| JP (1) | JP3524980B2 (en) |
| CN (1) | CN1151372C (en) |
| CA (1) | CA2189360C (en) |
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| JPS61264250A (en) * | 1985-05-17 | 1986-11-22 | Nippon Denso Co Ltd | Nitrogen oxide detector |
| US5037525A (en) * | 1985-10-29 | 1991-08-06 | Commonwealth Scientific And Industrial Research Organisation | Composite electrodes for use in solid electrolyte devices |
| JP2636883B2 (en) * | 1988-04-30 | 1997-07-30 | 日本碍子株式会社 | NOx concentration measuring device |
| DE4225775C2 (en) * | 1992-08-04 | 2002-09-19 | Heraeus Electro Nite Int | Arrangement for continuously monitoring the concentration of NO in gas mixtures |
| US5397442A (en) | 1994-03-09 | 1995-03-14 | Gas Research Institute | Sensor and method for accurately measuring concentrations of oxide compounds in gas mixtures |
| JP3296092B2 (en) * | 1994-06-20 | 2002-06-24 | 株式会社豊田中央研究所 | Nitrogen oxide sensor |
| JPH0862174A (en) * | 1994-08-19 | 1996-03-08 | Riken Corp | Nox sensor |
-
1995
- 1995-03-10 JP JP07842795A patent/JP3524980B2/en not_active Expired - Fee Related
-
1996
- 1996-03-06 CN CNB961904372A patent/CN1151372C/en not_active Expired - Fee Related
- 1996-03-06 CA CA002189360A patent/CA2189360C/en not_active Expired - Fee Related
- 1996-03-06 EP EP96905004A patent/EP0759552B1/en not_active Expired - Lifetime
- 1996-03-06 DE DE69632510T patent/DE69632510T2/en not_active Expired - Fee Related
- 1996-03-06 WO PCT/JP1996/000537 patent/WO1996028722A1/en active IP Right Grant
-
1998
- 1998-09-25 US US09/160,344 patent/US6019881A/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| EP0759552A1 (en) | 1997-02-26 |
| US6019881A (en) | 2000-02-01 |
| WO1996028722A1 (en) | 1996-09-19 |
| EP0759552B1 (en) | 2004-05-19 |
| JPH08247992A (en) | 1996-09-27 |
| CN1152352A (en) | 1997-06-18 |
| CN1151372C (en) | 2004-05-26 |
| DE69632510T2 (en) | 2005-05-12 |
| DE69632510D1 (en) | 2004-06-24 |
| CA2189360A1 (en) | 1996-09-19 |
| JP3524980B2 (en) | 2004-05-10 |
| EP0759552A4 (en) | 1997-05-28 |
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| EEER | Examination request | ||
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